Conventionally, a disaster prevention monitoring system that monitors for abnormalities such as fire or gas leakage by connecting a sensor such as a fire detector or gas detector to a transmission line of a receiver has been put to practical use. In this disaster prevention monitoring system, a digital signal serving as a downward telegraphic message of for example, control information is transmitted from the receiver to a terminal (sensor) in a voltage mode. On the other hand, a digital signal serving as an upward telegraphic message of, for example, sensor information is transmitted from the terminal to the receiver in the current mode.
FIG. 11 shows a transmission system 100 used for conventional disaster prevention monitoring (for example, refer to Patent Document 1).
A pair of transmission lines 112a and 112b also functioning as power supply lines are led out from a transmission output circuit 116 provided in a receiver 110 toward a monitoring terminal side. For example, sensors 114 (114a, 114b, 114c) having a digital transmission function are used as a plurality of monitoring terminals, and these sensors 114 are connected to the pair of transmission lines 112a and 112b. 
Incidentally, in the transmission system shown in FIG. 11, if a digital signal comprising a rectangular pulse train is transmitted from the transmission output circuit 116 to the sensors 114 without performing impedance matching, the signal is reflected and travels back and forth between the transmission ends and the last ends of the pair of transmission lines 112a and 112b, that is, between the transmission output circuit 116 and the sensor 114c. As a result, as shown in the waveform in FIG. 12, ringing occurs in the digital signal at the terminal end side of the pair of transmission lines 112a and 112b. Therefore, transmission and reception of the digital signal cannot be performed normally between the receiver 110 and the sensors 114.
Conventionally as a countermeasure against this problem, as shown in FIG. 13, a resistor R3 having a resistance value equal to a characteristic impedance Z of the pair of transmission lines 112a and 112b is inserted into the sensor 114c connected to the terminal ends of the pair of transmission lines 112a and 112b, to perform impedance matching. As a result, as shown in FIG. 14, waveform ringing in the digital signal can be suppressed on the terminal end side of the pair of transmission lines 112a and 112b.     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H09-91576